A number of commercial ethylene oxide processes utilize a tubular reactor for converting ethylene to ethylene oxide. This fixed bed reactor typically utilizes a silver-based catalyst which has been supported on a porous support and which is typically promoted with one or more alkali metal promoter(s). The shell side of the ethylene oxide reactor typically utilizes a high temperature coolant to remove the heat generated by the oxidation reaction. Under operating conditions a chlorohydrocarbon moderator is utilized to control the oxidation reaction. Reactor product gases are passed through an ethylene oxide absorber and the overhead gases from the absorber, containing unreacted ethylene and ballast gas such as methane and other inerts are recycled back to the reactor with some carbon dioxide and inerts in the overhead stream being removed on the way back to the reactor.
The usual practice for starting up fresh silver/alkali metal-supported ethylene oxide catalysts in a commercial plant is to first add ethylene and diluent gas; then slowly introduce oxygen to get the reaction started; and then to gradually introduce chlorohydrocarbon moderator to control the reaction after it is producing enough heat to become self-sustaining. For the traditional silver-based, alkali-metal promoted supported catalyst, the chlorohydrocarbon moderator serves to decrease the activity (i.e., raise the temperature as required to obtain a given conversion level) while increasing selectivity to ethylene oxide. When utilizing conventional alkali metal-promoted, supported silver catalysts, the catalysts are very active at normal start-up temperatures. Chlorohydrocarbon moderator levels are introduced after start-up to control the high catalyst activity to reduce the conversion level, and to prevent a "run away".
The appearance of a new generation of ethylene oxide catalysts comprising silver/rhenium/alkali metal supported on alumina has presented start-up considerations considerably different than those presented by the conventional silver/alkali metal catalysts. As is illustrated in FIG. 1, the rhenium-containing catalysts have a completely opposite activity response to the presence of chloride-containing moderator than do the conventional catalysts. The rhenium-containing catalysts have an initial low activity, requiring a very high reactor temperature (as measured by the reactor coolant temperature) to operate properly. Since most commercial reactors cannot reach this required high temperature during start-up, special techniques have been evolved. U.S. Pat. No. 4,874,879, issued Oct. 17, 1989, discloses a method of prechloriding these rhenium-containing catalysts to enhance their activity and allow start-up at low temperatures. The prechloriding enhances the initial activity of the rhenium-containing catalyst. There is no indication in this patent that such prechloriding allows for a faster start-up or enhances the life of such rhenium-containing catalyst.
It has been found that the application of a prechloriding technique to the conventional silver/alkali metal-promoted alumina-supported catalyst, while decreasing the activity of the catalyst, allows for a faster start-up of the reactor, thus providing a significant cost advantage as well as extending the life of the catalyst.